Interpretive Summary: The Nitrogen Index has been calibrated and validated across several regions. The Nitrogen Index can accurately assess the fate and transport of nitrogen under many different scenarios. The Nitrogen Index has been found to accurately predict the higher risk of nitrogen loss for scenarios of excessive nitrogen applications. The tool has been found to provide users with quick outputs that can aid decision-making regarding application of nitrogen fertilizer and manure management. The Nitrogen Index 4.5 prototype has been expanded and improved with new features. A phosphorus index and a new subroutine to assess direct and indirect N2O emissions have been added to version 4.5. Additionally, a new subroutine to generate recommendations for nitrogen, phosphorous and potassium fertilizer applications has been added for Mexico. The subroutine to generate recommendations for nitrogen inputs (organic and inorganic) has been added to all regions. The new Nitrogen Index 4.5 is expected to be released sometime in November/December 2013 (will be available in metric and English units), followed by the release of the new Nitrogen Index 4.5 smartphone application for Android™ systems soon afterwards. This tool can help users evaluate nitrogen management practices and assess the risk of nitrogen losses to increase nitrogen use efficiency of agricultural systems.

Technical Abstract:
Continued population growth creates a need for increased productivity of agricultural systems around the world. Increased agricultural productivity will be needed to support a population that is anticipated to have an additional 2.5 billion people by the year 2050. Nitrogen was part of the 20th century’s green revolution. Nitrogen continues to be essential for the viability and sustainability of agricultural systems that are important for feeding the growing global population. It is well known that nitrogen is a limiting factor for many agricultural systems, and that nitrogen inputs contribute to increased productivity. However, when more nitrogen is applied than necessary, increased losses of reactive nitrogen from agricultural systems occur, losses that can negatively impact the environment. Although there are always losses of nitrogen from agricultural systems via nitrate leaching, denitrification, emissions of the trace gas nitrous oxide (N2O), ammonia volatilization, and surface runoff (among other pathways), when nitrogen is applied at rates that are higher than needed, losses via these pathways have the potential to increase significantly. User-friendly, quick-to-use nitrogen tools can be used to help us evaluate nitrogen management practices and the potential risk of nitrogen losses. The Nitrogen Index is a simple, Tier-one tool that has the potential to help nutrient managers conduct quick assessments of nitrogen management practices and to help managers make decisions that can potentially reduce the risk of losses of reactive nitrogen to the environment and increase nitrogen use efficiency. The Nitrogen Index is similar to the P Index, and uses a series of qualitative and quantitative factors to assess the potential risk of nitrogen losses to the environment. The Nitrogen Index tool/approach is being used in different areas and can be adapted to a given region, state, and/or country.